This invention relates to a method of forming a colored or electrically conductive coating layer on a glass sheet by using a paste or paint containing an inorganic pigment or a metal powder together with a powdered glass and an organic binder.
It is conventional to form a colored coating layer on a glass sheet by applying a paste or paint prepared by dispersing an inorganic pigment and a finely powdered glass frit of a relatively low melting temperature in a mixed solution of an organic binder such as ethyl cellulose and an organic solvent such as pine oil or terpineol onto a surface of the glass sheet, usually by utilizing the technique of screen-printing, so as to form a paint layer of a suitable thickness on the glass surface, drying the printed paint layer and firing the paint-applid glass sheet to fuse the glass frit contained in the paint layer. An electrically conductive coating layer also can be formed by the same method by using a paint containing a suitable metal powder in place of the aforementioned pigment. If desired, a colored and conductive coating layer can be formed by using a paint containing both a pigment and a conducting metal powder in an adequated proportion.
In industrial practice of this coating method, drying of the printed paint layer is indispensable in order to facilitate handling of the paint-applied glass sheet before firing. As a matter of inconvenience, however, it takes a considerably long period of time to complete the drying though it is performed at an elevated temperature, because a solvent having a relatively high boiling point is used in the paint with a view to accomplishing printing of the paint on the glass sheet with accurate control of the thickness of the printed paint layer and without suffering from defects in the printed paint layer such as blurring. Particularly in the case of a multi-layer printing for the purpose of forming a multicolored coating layer, the accompaniment of a drying procedure to each printing procedure becomes a significant cause of low productivity of the coating process.
In view of such inconveniences, recently it has been proposed and already put into practice to use a photosensitive polymer as the organic binder in a paint used in the above described coating method. More particularly, use is made of an ultraviolet-sensitive unsaturated polymer which can rapidly be cured by radiation of ultraviolet rays. In the coating method using a paint containing such an unsaturated polymer as binder, exposure of the printed paint layer to ultraviolet rays soon results in cross linking of the unsaturated polymer in the paint layer so that the paint layer becomes hard and untacky. Therefore, the object of drying of the printed paint layer can be accomplished in a greatly shortened period of time.
According to our recognition, however, the use of a paint containing an ultraviolet-sensitive unsaturated polymer tends to result in insufficient strength of adhesion of the colored or conductive coating layer to the glass sheet. In the coating method of the described type, it is necessary that the organic binder contained in the paint, whether a familiar one such as ethyl cellulose or an ultraviolet-sensitive unsaturated polymer, be completely burnt out during the firing procedure. In the case of using the unsaturated polymer, however, the cross linking of the polymer caused by the radiation of ultraviolet rays makes it difficult to completely burn and gasify this polymer in the cured state, and therefore the coating layer after the firing and cooling tends to retain some carbon, which adversely influences the strength of adhesion between the coating layer and the glass sheet and, in an extreme case, becomes a cause of peeling of the coating layer from the glass sheet. Besides, the carbon retained in the coating layer produces an undesirable variation in the color tone of the coating layer, and this becomes a serious matter of disadvantage in the case of a colored coating layer.